This invention relates generally to gas turbine engines and more particularly, to engine bleed air systems used with gas turbine engines.
Gas turbine engines used in aircraft propulsion systems typically include a bleed air system that routes air from a compressor section of the engine to an environmental control system (ECS) on the aircraft. For example, in at least some engines, bleed air is bled from the compressor through holes or ports formed in the compressor housing. However, bleed air routed from the compressor section is generally at a higher pressure and temperature than desired for use by the ECS, and is therefore conditioned prior to use. More specifically, at least some known ECS include various components, such as regulating valves and heat exchangers, to condition the bleed air for use by the ECS.
Because the pressure of air bled from a specific bleed port may change significantly as engine operating conditions change, it may not be possible, without undue complexity and costs, to provide exactly the correct pressure to the ECS from the same bleed port. Accordingly, at least some known compressors include a plurality of bleed ports positioned at more than one location in the compressor, and also a plurality of external valves to control the flow of bleed air. Typically, such control valves include at least one check valve that includes flapper doors that permit air flow in only one direction through the doors when the doors are in an open position. When closed, the doors inhibit air flow in the opposite direction. Typically, the flapper doors pivot or rotate on a pin that extends across the valve.
During engine operation, the check valves may be subjected to vibrational stresses induced by the engine and/or excitation from the bleed air stream. For example, with known check valves, when the check valve flapper doors are open, the doors may be subjected to a flutter condition that may cause the flapper doors to vibrate against a stop. Over time, continued exposure to the vibrational stresses may damage the valve pin and/or may limit the useful life of the check valve.